1. Field of the Invention
The present invention relates to a group III nitride compound semiconductor device.
The present application is based on Japanese Patent Applications No. Hei. 11-276556, 2000-41222, 2000-191779 which are incorporated herein by reference.
2. Description of the Related Art
A group III nitride compound semiconductor device is used in a light-emitting device such as a light-emitting diode, etc. Such a light-emitting device has a configuration in which a group III nitride compound semiconductor layer having a device function is epitaxially grown on a surface of a substrate, for example, formed of sapphire.
The internal stress is, however, generated between the sapphire substrate and the group III nitride semiconductor layer because the sapphire substrate is different in thermal expansion coefficient and lattice constant from the group III nitride compound semiconductor layer. As a phenomenon caused by the internal stress, a bowing occurs in a growth of the group III nitride compound semiconductor on the sapphire substrate. If the bowing becomes too large, the crystallinity of semiconductor not only may be spoiled, but that the semiconductor layer may also has many cracks. And inconvenience occurs in alignment of photolithography at the time of production of the device.
In the background art, therefore, a so-called low-temperature buffer layer was formed between the substrate and the group III nitride compound semiconductor layer to thereby relax the internal stress.
The growth temperature of the group III nitride compound semiconductor layer for forming a device by a general metal organic chemical vapor deposition method (hereinafter referred to as xe2x80x9cMOCVDxe2x80x9d method) is 1000xc2x0 C. or higher. On the other hand, the growth temperature of the low-temperature internal stress layer is approximately in a range of from 400 to 500xc2x0 C. Hence, the temperature history of from the step of cleaning the substrate at about 1000xc2x0 C. to the growth of the group III nitride compound semiconductor layer is high temperature (1000xc2x0 C.)xe2x86x92low temperature (400 to 500xc2x0 C.)xe2x86x92high temperature (1000xc2x0 C.). Hence, not only was it difficult to control the temperature but also thermal efficiency was poor.
It may be, therefore, conceived that the buffer layer is formed at a high temperature. The problem of bowing, however, occurs again if a group III nitride compound semiconductor (for example, an AlN layer the same as the low-temperature buffer layer) is grown directly on the substrate at a high temperature of about 1000xc2x0 C.
The inventors of the present invention have made investigation over and over again to solve the problem of bowing. As a result, the inventors thought up the present invention as follows:
A group III nitride compound semiconductor device comprises an undercoat layer having a surface on which a group III nitride compound semiconductor layer having a device function can be formed, the surface of the undercoat layer containing inclined faces, wherein the projected area ratio of the inclined faces to the whole surface of the undercoat layer on a plane of projection is in a range of from 5 to 100%.
According to another aspect, preferably, the undercoat layer containing inclined faces is formed as a texture structure. Here, the xe2x80x9ctexture structurexe2x80x9d means a structure in which the surface of the undercoat layer is shaped like teeth of a saw in any sectional view, that is, a combination of a peak and a trough is repeated through an inclined face. The peaks may include those which are independent of each other as polygonal pyramids (inclusive of cones) or those which are standing in a row like a mountain range.
In this specification, a xe2x80x9csectional trapezoid shapexe2x80x9d means a shape in which there is a flat region at the top of each peak, and a shape in which the flat region is wider is referred to as a xe2x80x9cpit shapexe2x80x9d.
In this specification, when the projected area ratio occupied by the inclined face region on a whole plane of projection is in a range of from 70 to 100%, the shape is referred to as a xe2x80x9ctexture structurexe2x80x9d; when the projected area ratio is in a range of from 30 to 70%, the shape is referred to as a xe2x80x9csectional trapezoid shapexe2x80x9d; and when the projected area ratio is in a range of from 5 to 30%, the shape is referred to as a xe2x80x9cpit shapexe2x80x9d.
Use of the aforementioned undercoat layer relaxes internal stress between the group III nitride compound semiconductor layer and the substrate including the undercoat layer. It is supposed that internal stress applied to a hetero interface is relaxed by acting in directions parallel to the inclined faces because of the presence of the inclined faces in the hetero interface. If internal stress is relaxed in the aforementioned manner, the problem of bowing is reduced. As a result, not only can the group III nitride compound semiconductor layer be prevented from cracking, but also the crystallinity of the group III nitride compound semiconductor layer can be improved. Moreover, it becomes easy to perform alignment of photolithography at the time of production of the device.
Hereupon, the undercoat layer transmits light having a wavelength of not smaller than 360 nm because the undercoat layer is made of a group III nitride compound semiconductor. Incidentally, when the under coat layer is made of AlN (refractive index: 2.12) and the group III nitride compound semiconductor layer provided on the undercoat layer is made of GaN (refractive index: 2.60), the angle of incidence of light on the undercoat layer must be selected to be not larger than about 22 degrees so that light given from the GaN side is totally reflected on the undercoat layer. In the case of an undercoat layer having the aforementioned texture structure, it is impossible to obtain total reflection surely on the whole surface of the undercoat layer though it may be said that the reflectivity of the undercoat layer is relatively high because the angle of incidence of light on the surface of the undercoat layer becomes small.
From the above point of view, the present invention may be configured as follows.
A group III nitride compound semiconductor device comprises: a substrate; a group III nitride compound semiconductor layer having a function of light-emitting device or a function of a photodetector light-receiving device function; an undercoat layer formed between the substrate and the group III nitride compound semiconductor layer and made of a group III nitride compound semiconductor, the undercoat layer having a surface formed as a texture structure or shaped like trapezoids in section, or like pits; and a reflection layer formed on the surface of the undercoat layer and made of nitride of at least one kind of metal selected from the group consisting of titanium, zirconium, hafnium and tantalum, the reflection layer having a surface shape formed in accordance with the surface shape of the undercoat layer.
According to the group III nitride compound semiconductor device configured as described above, a reflection layer made of predetermined metal nitride is formed on the surface of the undercoat layer having a surface shape such as a texture structure, a trapezoid shape in section or a pit shape. The reflection layer also has such a surface shape as a texture structure, a trapezoid shape in section or a pit shape because the reflection layer is formed in accordance with the surface shape of the undercoat layer.
The reflection layer made of metal nitride has a so-called metallic-color mirror surface. Moreover, the angle of incidence of light on the surface of the texture structure, trapezoid shape in section or pit shape from the group III nitride compound semiconductor layer can be made smaller. Hence, the reflection layer according to the present invention can substantially totally reflect light incident on the reflection layer from the group III nitride compound semiconductor layer side.
The inventors of the present invention have already proposed nitride of at least one metal selected from the group consisting of titanium, zirconium, hafnium and tantalum so that a group III nitride compound semiconductor can be grown with good crystallinity on the metal nitride when the metal nitride is used as the aforementioned predetermined metal nitride (see Japanese Patent Publication No. Hei. 2000-323753). Also in the case where a group III nitride compound semiconductor is to be grown on a reflection layer made of the aforementioned metal nitride, internal stress between the group III nitride compound semiconductor layer and the substrate inclusive of the reflection layer and the undercoat layer can be relaxed because the surface of the reflection layer is formed as a texture structure, a trapezoid shape in section or a pit shape. As similar to the aforementioned, it is conceived that stress applied on a hetero interface is diffused in parallel to the inclined faces because of the presence of the inclined faces in the hetero interface and, accordingly, the stress can be relaxed. When internal stress is relaxed in the aforementioned manner, the bowing problem or cracking is reduced. Moreover, the crystallinity of the group III nitride compound semiconductor layer is improved, and the group III nitride compound semiconductor layer can be aligned easily when the device is produced.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.